Watch: US Senator John McCain diagnosed with brain cancer

Veteran US Senator John McCain, a former prisoner of war and the 2008 Republican presidential nominee, has been diagnosed with brain cancer, his office announced on Wednesday.

The 80-year-old lawmaker from Arizona underwent surgery to remove a blood clot from above his left eye last week and tests "revealed that a primary brain tumor known as a glioblastoma was associated with the blood clot," the Mayo Clinic, whose doctors performed the surgery in Phoenix, said in a statement released by McCain's office.

"The senator and his family are reviewing further treatment options," the statement added, noting they may include a combination of chemotherapy and radiation.

Glioblastoma is a particularly aggressive brain tumour affecting adults. It is of the same category of cancer that led to the death of another icon of the US Senate, Ted Kennedy, who died in 2009 at age 77.

It is not McCain's first bout with cancer. Doctors removed several malignant melanomas on McCain's skin in the 1990s and 2000s, including an invasive melanoma in 2000.

Experts had said this week that McCain's latest operation suggested the possibility of a return of cancer.

McCain's office said the senator "is in good spirits as he continues to recover at home with his family in Arizona," and that he will consult with his care team about when he might be able to return to work in Washington.

Senate Leader Mitch McConnell quickly issued a statement saying the entire chamber was praying for their colleague and hoped to see him back soon.

"John McCain is a hero to our conference and a hero to our country," McConnell said.

"He has never shied from a fight and I know that he will face this challenge with the same extraordinary courage that has characterized his life."

McCain, a former US Navy aviator, was shot down over Hanoi in 1967 as he conducted a bombing mission during the Vietnam War.

He was badly injured when he parachuted out, and was tortured during his five and a half years as a POW in Hanoi.

Released in 1973, McCain returned to the United States and won a US House seat in 1982 before winning election to the Senate four years later.​